The present invention is an improvement in a method and apparatus for oscillating the doctor blade used in a gravure type printing press.
In intaglio printing, the gravure cylinder, having a highly polished copper or steel surface etched or engraved with the design to be printed, rotates through a trough of ink, which is held on the surface as well as in the etched wells. As the cylinder continues its rotation, it passes under a doctor blade, which is a thin, flexible steel blade or scraper that extends the entire length of the cylinder and bears at an angle against it. The doctor blade wipes by scraping the printing cylinder surface clean to leave ink only in the etched walls. The ink left in the wells is then transferred to a paper web travelling between the gravure cylinder and a rubber impression roller pressing the web against the gravure cylinder. Optionally, a back up or pressure roller may be mounted in a tangential relationship with the impression roller for assuring that the proper pressure is exerted by the impression roller on the gravure cylinder to pull the ink out of the etched wells and onto the paper.
To minimize wear and the possible effects of small nicks, the doctor blade is made to oscillate lengthwise against the cylinder. Preferably, however, the blade is oscillated in non-repeating cycles, i.e. such that the blade does not repeat its exact motion each stroke. In comparison to drives in which the doctor blade moves in a repeating oscillation motion, a doctor blade drive producing non-repeat back and forth motion reduces doctor blade wear and the incidence of cracked doctor blades. The non-repeat feature also acts to dislodge foreign particles, such as paper lint, from under the doctor blade. If such particles lodge under the doctor blade, the printing cylinder may be damaged, the printing quality can be adversely affected, and press down time is increased. All these factors result in lost production.
In the past, simple oscillation systems have used mechanical devices such as eccentrics driven from the press drive or a hydraulic cylinder to produce a repeating push-pull motion. Doctor blades have also been oscillated by a mechanical drive operated off the press to produce a non-repeat oscillation. In another system producing a non-repeat oscillation, the doctor blade is driven back and forth by a combination of hydraulic, pneumatic and mechanical systems.
Non-repeat mechanical drives operated from the press are disadvantageous in that they may induce drive disturbances in the doctor blade motion affecting printing quality. Also, such mechanical drives connected to the press drive have a fixed speed ratio, related to the press drive rpm, and the rate of oscillation cannot be changed. The known non-repeat drive using the combined hydraulic, pneumatic and mechanical systems is complicated and expensive. It also requires considerable maintenance resulting in operators bypassing the non-repeat feature and using the system as a simple push-pull motion. The simple oscillating system driven from the press and using an eccentric control has the same shortcomings as the mechanical non-repeat systems described above.